Well, since this is your first rp I say limit it to the maximum of at least six sign ups. Speaking from experience, the posting goes much faster when there are less people and also you won't have to keep track of an endless amount of characters and constantly updating events for them. Start at capping the sign ups at six and once you feel more comfortable with GMing and you feel you can keep track of more people open up some more slots. Also, wouldn't hurt to advertise your rp on the discussion board to let more people know about it.